1. Field of the Invention
The present invention relates to an apparatus and method for estimating UpLink (UL) transmission timing in a wireless communication system. More particularly, the present invention relates to an apparatus and method for regulating UL transmission timing by estimating a time offset between DownLink (DL) reception frames in a Mobile Station (MS) of a wireless communication system.
2. Description of the Related Art
Typical examples of a 4th Generation (4G) communication system include an Institutes of Electrical and Electronics Engineers (IEEE) 802.16d system and an IEEE 802.16e system. The IEEE802.16 d/e system may be implemented based on Wimax, Wibro, or Mobile Wimax.
In the conventional Mobile Wimax (or Wibro) system, a Mobile Station (MS) tracks DownLink (DL) timing by using a DL preamble signal received for each frame so as to achieve a time synchronization between a Base Station (BS) and the MS, predicts UpLink (UL) transmission timing by using the tracked DL timing, and transmits an UL frame. When timing for receiving the UL frame does not coincide with UL frame reception timing of the BS, the BS transmits a ranging response message to the MS and thus instructs the MS to regulate the UL transmission timing.
FIG. 1 illustrates UL/DL transmission/reception timings between an MS and a BS.
In FIG. 1, the DL transmission timing of the BS is T1 101, and the UL reception timing of the BS is T2 103. In the BS, a DL period, a UL period, a Transmit Time Gap (TTG), and a Receive Time Gap (RTG) do not change, and the sum of respective time periods is 5 ms.
When the BS transmits a DL frame at the transmission timing T1 101 in step S1, the MS receives the DL frame after a propagation delay time elapses from the transmitting timing T1 101 in step S2. The propagation delay time may be defined as a half of a Round Trip Delay (RTD) time, that is, RTD/2.
Upon receiving the DL frame, the MS predicts a time interval T between a DL period and a UL period by using a DL preamble signal. Then, the MS transmits a UL frame to the BS at a time which is prior to the timing T2 103 by the propagation delay time (that is, RTD/2) in step S3. Accordingly, in step S4, the BS can receive the UL frame transmitted from the MS at the timing T2. After receiving the DL frame as shown in step A1, the MS predicts a time interval T between a DL period and a UL period, and transmits the UL frame after the time interval T elapses. Thus, the BS can correctly receive the UL frame at the UL reception timing T2 as shown in step A2.
When the MS moves apart from the BS in a state that the time interval T is predicted as described above, due to the movement of the MS, the MS receives the DL frame from the BS in step F1 after a propagation delay time D elapses, wherein the propagation delay time D is greater than the propagation delay time (that is, RTD/2) in step S2. Due to the movement of the MS, a time synchronization between the BS and the MS changes. However, since the UL timing is not predicted until a ranging response message is received, the MS transmits the UL frame in step F2 by using the previously predicted time interval T. Accordingly, the BS receives the UL frame at a time later than the original UL frame reception timing in step F3. Since the MS transmits the UL frame by using the time interval T even if a propagation delay time changes as shown in step B 1, the BS receives the UL frame at a time later than the original reception timing T2 103 as shown in step B2.
On the contrary, when the MS moves close to the BS, if the UL frame transmission timing is determined by using the aforementioned time tracking method, the BS receives the UL frame at a time which is prior to the timing T2 103 at which the UL frame is received in practice.
As described above, a method for transmitting a UL frame by predicting UL transmission timing according to DL timing can correctly operate in an environment where an MS exists in a fixed location and thus a propagation delay and a channel state do not change. However, in an environment where the MS moves and thus the propagation delay and the channel state continuously change, the UL transmission timing cannot be correctly predicted.
In addition, changes in a channel state between the BS and the MS result in changes in signal strengths of signals received through paths having different delays in a multi-path scenario. Thus, a reference path, which is used by the MS to track the timing, may also change. For example, as shown in FIG. 2, when a path having a maximum signal strength changes from a first path to a second path, the reference path used for timing tracking is shifted by D.
As described above, in a method for predicting UL transmission timing in an MS, the UL transmission timing is regulated only when a ranging response message is received from a BS. Therefore, disadvantageously, the method cannot proactively deal with changes in a propagation delay or a channel state. Since the ranging response message is not received for each frame, the MS uses incorrectly predicted UL transmission timing until the ranging response message is received. As a result, the BS cannot receive a UL transmission frame at correct timing. Accordingly, link throughput deteriorates, which leads to overall quality deterioration.
Accordingly, there is a need for an apparatus and method for regulating UL transmission timing by estimating a time offset between DownLink (DL) reception frames in a Mobile Station (MS) of a wireless communication system.